1. Field of the Invention
This invention relates generally to multistage hydraulic machines provided with a plurality of runners, such as multistage pumps, multistage turbines and multistage pump-turbines, and more particularly, to a multistage hydraulic machine capable of performing a safety operation by virtue of respectively restraining water thrusts developed in respective stages.
2. Description of the Prior Art
In a Francis-type hydraulic machine, axial water thrusts acting on runners thereof are generally developed by an axial imbalance of water pressures acting on respective water-contact surfaces of the runners. However, most of the axial water thrust is developed not by water pressure acting on the surfaces of runner passages surrounded by runner vanes, runner crowns and runner bands, but rather by an axial imbalance of water pressure acting on respective water-contact surfaces of the back portions of the runner crowns and bands which are rearward with respect to the runner passage surface. The pressure effected area of the runner crown backside is greater than the pressure-effected area of the runner band backside by an area which is nearly equal to the area of the runner outlet portion so that downwardly directed water pressure acting on the runner crown backside becomes greater than upwardly directed water pressure acting on the runner band backside. It is, therefore, commonly experienced that with respect to the whole runner, a downward-axial thrust heading from the crown side to the band side is developed.
In smaller capacity, smaller sized hydraulic machines, such axial water thrust can be supported by the externally disposed thrust bearing. However, in medium or larger-capacity hydraulic machines, such water thrust becomes so much greater that a thrust bearing to support such axial water thrusts is impossible to manufacture and thus it becomes necessary to alleviate such axial water thrusts in an appropriate manner.
Therefore, hitherto in a single-stage hydraulic machine it has been common practice to dispose an intermediate seal on the runner crown backside so that a runner back chamber which is positioned between the runner crown and the head cover can be partitioned to from an inner and outer back chamber, and further to utilize an inner equalizing pipe or a balancing hole which communicates with a runner outlet portion or a draft tube portion, respectively, at the inner back chamber. This construction permits lowering of the water pressure of the inner back chamber with respect to the water pressure level at the runner outlet.
As the water pressure of the inner back chamber is acting on the runner crown backside, the water thrust caused by the water pressure of the inner back chamber, which is acting downwardly on the runner, is reduced by the above-mentioned construction. Therefore, the water thrust acting on the whole runner is also reduced by the above-mentioned construction.
Also in the case of a multistage hydraulic machine, in terms of the runner of the highest-pressure stage thereof being the same as in the above-described singlestage hydraulic machine, it is possible to dispose an intermediate seal on the runner backside portion, and further to dispose an inner equalizing pipe which communicates with lower-pressure flow passage at the inner back chamber so that the hydraulic pressure therein is lowered. However, in terms of the runners of lower-pressure stages except the highest-pressure stage, the runner backside portions are situated in the center portion of hydraulic machine, which is extremely complicated in construction, so that there are encountered many difficulties such that an inner equalizing pipe can hardly be utilized, and even if an intermediate seal could be disposed on the runner crown backside, an appropriate seal clearance thereof can scarcely be established and maintained. Thus, it is almost impossible to carry out sufficient maintenance and inspections without dismantlement of the entire machine such that there are many impractical aspects in the device described above. Accordingly, it has been a common practice not to dispose an intermediate seal on the runner crown backside but instead to inevitably support the developed water thrust by means of an externally disposed thrust bearing.
Therefore, multistage hydraulic machines have inevitably had to utilize larger capacity thrust bearings, and this has been a great obstacle to development of larger capacity multistage hydraulic machines which produce excessively larger capacity hydraulic thrusts.